Hello bristolflyer.
Haven't got any detailed information re number of passengers who connect at AMS, CDG, EWR or even at BRU. However, in the past it's often been reported that a high proportion of passengers fly on from AMS - I've seen some figures put it at 60% on occasions.
The AMS route has stood up well this summer after Cityhopper
reduced the number of weekday rotations from 4 to 3. Passenger numbers on the route as a whole for the four summer months so far reported (April to July) show a drop of only 3.1% - 81,521 passengers flown this year compared to 84,113 in the same period in 2008.
Unless easyJet's share has risen dramatically, it can be assumed that KLM has held on to a substantial number of the passengers even with one less daily rotation.
It also has to be noted that this summer for the first time Ryanair has flown 3 x weekly to Eindhoven and almost certainly taken some passengers who would have flown to AMS, although there can't be much scope for transfer traffic at Eindhoven.
BRS to CDG carried 56,567 passengers from April to July this year and 67,226 in summer 2008. Whilst AF's rotations remained the same easyJet was only single daily whereas last year it was double daily on Mondays, Wednesdays and Fridays for much of the summer and single daily for the rest of the week. The loss of these rotations almost certainly accounts for the lower figure this summer.
Usual caveat re yields of course.
As for Hamburg, easyJet did the route for about a year in 2005-2006 but passenger numbers, though never an absolute disaster, were not up to the level of most of easyJet's other BRS routes then. I think the best months only had around 70% load factors.
I take the point about most of the French routes being leisure routes though CDG which you mentioned and Toulouse might be part exceptions, even though there is a daily TLS from Filton 'for the trade'. I note that for the coming winter Ryanair has suspended (hope that's what it is) nearly all the new summer routes brought in this year, with most being holiday routes to France and Italy.
As discussed in the Ryanair thread, that airline seems to have axed a susbstantial number of routes from several UK airports in the winter ahead, although BRS will have seven new winter Ryanair routes (some started in the summer) - Tenerife, Gran Canaria, Lanzarote, Alicante, Malaga, Reus and Malta.
Although there are gaps in the current winter schedule all four based aircraft will be needed at least for part of every day and there are also 24 weekly rotations currently listed using non-based aircraft.
Part of the reduction in based unit rotation numbers is due to some of the new routes that take between seven and nine hours to complete there and back.
Unless there are more routes pulled Ryanair will still be busier than last winter when they had just the two based aircraft.